Investigators

Period of Record

July 1993 - December 2001

Methods

The listed data have been obtained from flask air samples returned to the CSIRO GASLAB for analysis. Typical sample storage times range from days to weeks for some sites (e.g., Cape Grim) to as much as one year for Macquarie Island and the Antarctic sites. Experiments carried out to test for any change in sample CH4 mixing ratio during storage have shown no drift to within detection limits over test periods of several months to years (Cooper et al., 1999).

Samples were analysed by gas chromatography with flame ionization detection (FID). Three individual but similarly configured Carle gas chromatographs were used over the length of the record. They are labeled "Carle-1" (C1), "Carle-2" (C2) and "Carle-3" (C3). Further details of CSIRO's global sampling network, sampling and analytical techniques are provided by Francey et al. (1996); measurement uncertainty is discussed by Langenfelds et al. (2001).

Data are reported in the CSIRO94 CH4 scale (Steele et al., 1996), which is derived from the CH4 scale maintained at NOAA/CMDL (Dlugokencky et al., 1994). The link to this scale was established with two high pressure cylinders containing dry, natural air that were calibrated by NOAA/CMDL between 1987 and 1990. Results from later exchanges of high-pressure cylinders indicate a small, but measurable difference (0.36 ±0.17 ppbv). Stability of the CSIRO scale is monitored with approximately 25 assorted long-lived standards. More detailed calibration information is given by Langenfelds et al. (2001). Comparison of CSIRO and NOAA flask measurements indicates agreement to within 1 ppbv (Masarie et al., 2001).

These data represent monthly means, calculated as the mean of daily values from a smooth curve fit to the data using the curve-fitting routines described by Thoning et al. (1989).

Estevan Point, BC, Canada
49°23' N, 126°32' W,
39 m above MSL

Trends

An annual pattern in atmospheric CH4 is evident, largely due to an increase in its destruction by the OH radical during the summer months. Additional influences include spatial and seasonal variations in source strength associated with varying trajectories of arriving air at different times of the year. An unsteady increase, averaging around 4 ppbv/year since 1994, includes decreases for 3 years and increases for 4 years. Peak values occurred in the winter of 1998-99.